Methods for the flow-cytometric assay of radioactivity of labelled cells are being developed. Flow-autoradiography used in conjunction with conventional flow-cytometry should permit correlated assay of rates of synthesis as well as content of macromolecules in cells. The method will be one of several applied to the study of fluctuations of rates of DNA synthesis during the S phase of proliferating cells. Dual parameter analysis of porphyrin and DNA of erythroblasts is being done, porphyrin used as an index of differentiation to resolve successive erythroblast generations, permitting comparative analysis of their cell cycle kinetics as the cells approach terminal differentiation.